Three U.S. nuclear power plants need increased oversight from federal regulators because of safety problems or unplanned shutdowns, the Nuclear Regulatory Commission said Thursday, although officials said all are operating safely.

NRC Chairman Gregory Jaczko said the three plants — in South Carolina, Kansas and Nebraska — "are the plants we are most concerned about" among the 65 U.S. nuclear power plants in 31 states.

Jaczko did not identify the plants, but an agency spokesman said they are the H.B. Robinson nuclear plant in South Carolina, Fort Calhoun in Nebraska and Wolf Creek in Kansas.